Gene transfer with viral vectors has the potential to correct defects associated with genetic diseases such as cystic fibrosis. Because of their ability to efficiently and stably integrate their genomes into the host chromosome retroviruses are ideal vectors for gene transfer. Epithelial cells lining the upper airways represent the appropriate targets for viral delivered genes. However, in vitro and in vivo studies using viral vectors suggest that gene transfer into differentiated airway epithelia with viral vectors is inefficient. One obstacle that appears to severely limit gene transfer into airway epithelial cells by viral vectors is inefficient entry from the apical surface. To circumvent this obstacle, we are developing a unique system for targeted viral infection utilizing influenza hemagglutinin (HA) to mediate viral entry. For this targeting system we have produced a multiply mutated variant of HA that no longer binds its cellular receptor but which can efficiently mediate membrane fusion upon endocytosis. Co-incorporation of a targeting ligand with the mutant HA directs efficient infection of viruses to receptor expressing cells. We will target HIV-1 based retroviral vectors to airway epithelial cells using this novel system to achieve stable gene transfer. To accomplish this task we propose the following specific aims: 1) Define epithelial cell apical surface molecules to utilize for targeting by screening phage display libraries, 2) produce HIV-1 vectors carrying HA and airway epithelial cell targeting ligands and develop two novel systems for incorporation of ligands into virions 3) analyze targeting and infection by these HIV-1 vectors using in vitro and in vivo models of airway epithelia.